diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 0505615..7ba8a0b 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -11,7 +11,6 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
 use Drupal\Core\Datetime\DrupalDateTime;
-use Drupal\Core\Template\Attribute;
 use Drupal\system\Form\ModulesInstallConfirmForm;
 use Drupal\system\Form\ModulesUninstallConfirmForm;
 
@@ -1402,7 +1401,7 @@ function template_preprocess_admin_block(&$variables) {
 function template_preprocess_admin_block_content(&$variables) {
   if (!empty($variables['content'])) {
     $compact = system_admin_compact_mode();
-    $variables['attributes'] = new Attribute(array('class' => array('admin-list')));
+    $variables['attributes'] = array('class' => array('admin-list'));
     if ($compact) {
       $variables['attributes']['class'][] = 'compact';
     }
@@ -1685,8 +1684,6 @@ function template_preprocess_system_themes_page(&$variables) {
   $groups = array();
   $theme_groups = $variables['theme_groups'];
 
-  // Top level attributes.
-  $variables['attributes'] = !empty($variables['attributes']) ? new Attribute($variables['attributes']) : new Attribute();
   // We have a default class variable that we don't want here.
   // @see http://drupal.org/node/1938430
   unset($variables['attributes']['class']);
@@ -1702,7 +1699,7 @@ function template_preprocess_system_themes_page(&$variables) {
     $theme_group['state'] = $state;
     $theme_group['title'] = $title;
     $theme_group['themes'] = array();
-    $theme_group['attributes'] = new Attribute(array('class' => array('system-themes-list', 'system-themes-list-' . $state, 'clearfix')));
+    $theme_group['attributes'] = array('class' => array('system-themes-list', 'system-themes-list-' . $state, 'clearfix'));
 
     foreach ($theme_groups[$state] as $theme) {
       $current_theme = array();
@@ -1724,7 +1721,7 @@ function template_preprocess_system_themes_page(&$variables) {
       // Style theme info
       $theme->classes[] = 'theme-selector';
       $theme->classes[] = 'clearfix';
-      $current_theme['attributes'] = new Attribute(array('class' => $theme->classes));
+      $current_theme['attributes'] = array('class' => $theme->classes);
       $current_theme['name'] = $theme->info['name'];
       $current_theme['version'] = isset($theme->info['version']) ? $theme->info['version'] : '';
       $current_theme['notes'] = count($theme->notes) ? '(' . join(', ', $theme->notes) . ')' : '';
@@ -1749,7 +1746,7 @@ function template_preprocess_system_themes_page(&$variables) {
         $current_theme['compatibility'] = array(
           '#theme' => 'links',
           '#links' => $theme->operations,
-          '#attributes' => new Attribute(array('class' => array('operations', 'clearfix'))),
+          '#attributes' => array('class' => array('operations', 'clearfix')),
         );
       }
       $theme_group['themes'][] = $current_theme;
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 9d4fdd7..187b690 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -140,9 +140,9 @@ function system_theme() {
   return array_merge(drupal_common_theme(), array(
     'system_themes_page' => array(
       'variables' => array(
-        'theme_groups' => array(),
-        'theme_group_titles' => array(),
-        'admin_theme_options' => array(),
+        'theme_groups' => NULL,
+        'theme_group_titles' => NULL,
+        'admin_theme_options' => NULL,
       ),
       'file' => 'system.admin.inc',
       'template' => 'system-themes-page',
@@ -162,36 +162,36 @@ function system_theme() {
       'file' => 'system.admin.inc',
     ),
     'status_report' => array(
-      'variables' => array('requirements' => array()),
+      'variables' => array('requirements' => NULL),
       'file' => 'system.admin.inc',
       'template' => 'status-report'
     ),
     'admin_page' => array(
-      'variables' => array('blocks' => array()),
+      'variables' => array('blocks' => NULL),
       'file' => 'system.admin.inc',
       'template' => 'admin-page',
     ),
     'admin_block' => array(
-      'variables' => array('block' => array()),
+      'variables' => array('block' => NULL),
       'file' => 'system.admin.inc',
       'template' => 'admin-block',
     ),
     'admin_block_content' => array(
-      'variables' => array('content' => array()),
+      'variables' => array('content' => NULL),
       'file' => 'system.admin.inc',
       'template' => 'admin-block-content',
     ),
     'system_admin_index' => array(
-      'variables' => array('menu_items' => array()),
+      'variables' => array('menu_items' => NULL),
       'file' => 'system.admin.inc',
       'template' => 'system-admin-index',
     ),
     'system_powered_by' => array(
-      'variables' => array(),
+      'variables' => NULL,
       'template' => 'system-powered-by',
     ),
     'system_compact_link' => array(
-      'variables' => array(),
+      'variables' => NULL,
       'template' => 'system-compact-link',
     ),
     'system_date_format_localize_form' => array(
@@ -3523,19 +3523,6 @@ function system_timezone($abbreviation = '', $offset = -1, $is_daylight_saving_t
 }
 
 /**
- * Prepares variables for the Powered by Drupal template.
- *
- * Default template: system-powered-by.html.twig.
- *
- * This doesn't do anything extra to prepare the template, but is given simply
- * for example purposes.
- *
- * @ingroup themeable
- */
-function template_preprocess_system_powered_by(&$variables) {
-}
-
-/**
  * Prepare variables for system compact link templates.
  *
  * Default template: system-compact-link.html.twig.
diff --git a/core/modules/system/templates/admin-block-content.html.twig b/core/modules/system/templates/admin-block-content.html.twig
index 80c0526..1900082 100644
--- a/core/modules/system/templates/admin-block-content.html.twig
+++ b/core/modules/system/templates/admin-block-content.html.twig
@@ -4,7 +4,7 @@
  * Default theme implementation for the content of an administrative block.
  *
  * Available variables:
- * - content: An array containing information about the block. Each element
+ * - content: A list containing information about the block. Each element
  *   of the array represents an administrative menu item, and must at least
  *   contain the keys 'title', 'href', and 'localized_options', which are
  *   passed to l(). A 'description' key may also be provided.
@@ -18,12 +18,12 @@
  */
 #}
 {% if content %}
-<dl{{ attributes }}>
-  {% for item in content %}
-    <dt>{{ item.link }}</dt>
-    {% if item.description %}
-      <dd>{{ item.description }}</dd>
-    {% endif %}
-  {% endfor %}
-</dl>
+  <dl{{ attributes }}>
+    {% for item in content %}
+      <dt>{{ item.link }}</dt>
+      {% if item.description %}
+        <dd>{{ item.description }}</dd>
+      {% endif %}
+    {% endfor %}
+  </dl>
 {% endif %}
diff --git a/core/modules/system/templates/admin-block.html.twig b/core/modules/system/templates/admin-block.html.twig
index 986b959..8e9bc96 100644
--- a/core/modules/system/templates/admin-block.html.twig
+++ b/core/modules/system/templates/admin-block.html.twig
@@ -18,9 +18,14 @@
  */
 #}
 {% if block.show %}
-<div class="admin-panel">
-  {% if block.title %}<h3>{{ block.title }}</h3>{% endif %}
-  {% if block.content %}<div class="body">{{ block.content }}</div>
-  {% elseif block.description %}<div class="description">{{ block.description }}</div>{% endif %}
-</div>
+  <div class="admin-panel">
+    {% if block.title %}
+      <h3>{{ block.title }}</h3>
+    {% endif %}
+    {% if block.content %}
+      <div class="body">{{ block.content }}</div>
+    {% elseif block.description %}
+      <div class="description">{{ block.description }}</div>
+    {% endif %}
+  </div>
 {% endif %}
diff --git a/core/modules/system/templates/admin-page.html.twig b/core/modules/system/templates/admin-page.html.twig
index ab16d6e..f86b283 100644
--- a/core/modules/system/templates/admin-page.html.twig
+++ b/core/modules/system/templates/admin-page.html.twig
@@ -5,7 +5,8 @@
  *
  * Available variables:
  * - system_compact_link: Themed link to toggle compact view.
- * - containers: An array of administrative blocks keyed by position: left or right.
+ * - containers: An list of administrative blocks keyed by position: left or
+ *   right.
  *
  * @see template_preprocess()
  * @see template_preprocess_admin_page()
diff --git a/core/modules/system/templates/status-report.html.twig b/core/modules/system/templates/status-report.html.twig
index f8b74d6..a95e3cf 100644
--- a/core/modules/system/templates/status-report.html.twig
+++ b/core/modules/system/templates/status-report.html.twig
@@ -4,7 +4,7 @@
  *
  * Available variables:
  * - requirements: contains multiple requirement instances. Each requirement
- *     has .title, .value and .description.
+ *   has .title, .value and .description.
  * - severity: The severity of error. Contains .title and .class.
  *
  * @see template_preprocess()
diff --git a/core/modules/system/templates/system-admin-index.html.twig b/core/modules/system/templates/system-admin-index.html.twig
index 54cf0e2..5c2dc24 100644
--- a/core/modules/system/templates/system-admin-index.html.twig
+++ b/core/modules/system/templates/system-admin-index.html.twig
@@ -12,7 +12,6 @@
  *
  * @ingroup themeable
  */
- @todo: remove this file once http://drupal.org/node/1842232 is resolved.
 #}
 <div class="admin clearfix">
   {{ system_compact_link }}
diff --git a/core/modules/system/templates/system-modules-details.html.twig b/core/modules/system/templates/system-modules-details.html.twig
index 6321b28..0f946c4 100644
--- a/core/modules/system/templates/system-modules-details.html.twig
+++ b/core/modules/system/templates/system-modules-details.html.twig
@@ -4,7 +4,7 @@
  * Default theme implementation to display the modules form.
  *
  * Available variables:
- * - content: file form element html.
+ * - content: File form element html.
  *
  * @see template_preprocess()
  * @see template_preprocess_system_modules_details()
diff --git a/core/modules/system/templates/system-themes-page.html.twig b/core/modules/system/templates/system-themes-page.html.twig
index c20544a..0035809 100644
--- a/core/modules/system/templates/system-themes-page.html.twig
+++ b/core/modules/system/templates/system-themes-page.html.twig
@@ -10,22 +10,22 @@
  * Each theme_groups[group] contains an array of theme groups.
  *
  * Each group in theme_groups[group] contains:
- * - group.attributes: Element attributes specific to this group.
- * - group.title: Title for the theme group.
- * - group.state: State of the theme group.
- * - group.themes: An array of themes within that group.
+ * - attributes: Element attributes specific to this group.
+ * - title: Title for the theme group.
+ * - state: State of the theme group.
+ * - themes: An array of themes within that group.
  *
  * Each group.themes[theme] contains an array of themes.
  *
  * Each theme in group.themes[theme] contains:
- * - theme.attributes: Element attributes specific to this theme.
- * - theme.screenshot: Render of theme screenshot.
- * - theme.description: Description of the theme.
- * - theme.name: Name of theme.
- * - theme.version: Verions number of theme.
- * - theme.notes: Identifies what context this theme is being used.
+ * - attributes: Element attributes specific to this theme.
+ * - screenshot: Render of theme screenshot.
+ * - description: Description of the theme.
+ * - name: Name of theme.
+ * - version: Verions number of theme.
+ * - notes: Identifies what context this theme is being used.
  *   eg. (default theme, admin theme)
- * - theme.compatibility: Description of any incompatibility issues,
+ * - compatibility: Description of any incompatibility issues,
  *   if the theme is compatible, provides a list of links.
  *
  * @see template_preprocess()